iOS and macOS is basically the same OS.
iOS uses the same kernel and main userland, and was developed just as a spin-off of OS X with more lockdown enabled, some inapplicable services removed, and a profile that fits smaller devices. Otherwise the kernel, fs, userland, etc is 90% the same, minus different APIs (mostly UI) on top (which they also try to unify for several releases now).
macOS+iOS should be seen as the same thing as far as system-level development (kernel, fs, etc) are concerned.
Of course features still need integration to the respective form factors and UI paradigms (e.g. APFS required userland and Disk Utility changes, for one), and Apple has done that for both platforms.
>Stability, usability, and polish
And yet, Snow Leopard was created exactly because people complained Leopard and earlier were not stable enough. It was the release to focus on stability and polish over new features (and they even admitted this was alluded in the name change, "Leopard" -> "Snow Leopard" as opposed to a whole new version-animal).
And if we go back then, we can find tons of complains about instability, lack of polish, the world ending, etc.
>They've shown themselves utterly incapable of releasing a quality update each year.
Given that they ship 100 million or so updates, they have almost immediate adoption (far quicker than new Windows version), and the complains always subdue after 1 point release or so, I don't think so.
Not to mention most complains now are about them deprecating stuff (32 bits for one) -- as opposed to breaking stuff.
Heck, they pushed a whole new FS to hundreds of millions of users (5x+ that with iOS in), and there hardly any issues to write home about -- and that in record time (FS experts said it normally takes 10 years to mature a new FS to do such a conversion and warned of apocalypse which didn't happen).